Sticky

I’ve mostly used yellow glue for making rockets so far. Specifically Elmer’s Carpenter’s Glue. More specifically an orange bottle of Elmer’s Carpenter’s Glue that I bought God only knows when but it’s been around a good long time. The stuff still works though. Sometimes too well.

That is, on things like installing motor mounts, or tube couplers. You smear some in the body tube, you go to insert the motor mount or coupler, and within moments after it hits the glue, the glue grabs it and it’s frozen in place. If you didn’t quite get it into the intended position by then, guess what? You are, ahem, stuck with it.

Another issue with yellow glue is a phenomenon called Coke-bottling. Evidently it shrinks as it dries, and in the case of motor mounts, it pulls the body tube in around the centering rings, creating a small deformation of the tube. I have to say I’ve never noticed Coke-bottling in my rockets, and even after I learned about it and went back to look at them, I can’t see it. Okay, maybe I see some very slight deformation; you’d have to look for it, very closely, to see it if I’m not completely imagining it. Maybe this means I’m not using enough glue? Because some people certainly have had problems with it.

So I like the yellow glue for things like building motor mounts, or installing fins; but for installing stuff into body tubes, not so much. What else is there?

There’s white glue. Specifically Elmer’s Glue-All. An old friend of the family. Back about a thousand years ago when I was growing up, Elmer’s Glue-All was what my dad always used for his woodworking projects. And if you do a search on the Rocketry Forum you’ll read that it doesn’t grab or Coke-bottle the way yellow glue does.

Only these days, if you go to buy Elmer’s Glue-All, you’ll discover two things:

  1. It’s not so easy to find! Or at least, our local Home Depot seems not to carry it. Yellow glue, lots of it, but no white. Lowe’s, which I generally disprefer to Home Depot, does have it, though.
  2. When you find it, on the label it says “NOW STRONGER FORMULA!” And back on the Rocketry Forum, you’ll find more recent threads according to which the new formula has much more tendency to grab than the old formula did.

What else? There’s Elmer’s School Glue, which at least one NOW STRONGER FORMULA Glue-All critic likes. Others don’t. There’s various flavors of Titebond yellow glue, which some say grabs less than Elmer’s. There’s Gorilla Glue, which some people like, some don’t. There’s epoxy.

I decided to run some tests. Ladies and gentlemen, our contestants:2013-08-07 06.54.20

First, a bottle of Elmer’s Glue-All (which I bought before reading about the NOW STRONGER FORMULA — and which I’m using to build a cardstock rocket.) Second, a new bottle of Elmer’s Carpenter’s Glue (which also says NOW STRONGER FORMULA. Well, it can hardly grab harder, so let’s not get upset over that.) I bought that just a couple weeks ago, so we’re comparing fresh stuff to fresh stuff here. Also bought recently, two kinds of epoxy. One is Devcon 5-minute; it comes in a double syringe for somewhat fool-resistant measuring of equal parts resin and hardener. (The home centers I looked in didn’t seem to have any clear epoxies in bottles.) The other is some 30-minute epoxy from our local hobby store; it has the store’s name on it, but it comes from Bob Smith. It’s in separate bottles, and measuring out equal quantities is up to you. (They also had the 5- and 15-minute varieties in the same kinds of bottles.)

Our first event is the purchasing event:

  1. Elmer’s Glue-All: Not available at Home Depot, at least not at ours when I looked. I found it at Lowe’s. (Ace / True Value / misc. mom and pop hardware stores are nonexistent on our side of town.) Online, Amazon has the 7.625 oz bottle for $4.63.
  2. Elmer’s Carpenter’s Glue: At Home Depot and Lowe’s. Amazon has the 8 oz bottle for $2.97.
  3. Devcon 5-Minute Epoxy: At Lowe’s. Lowe’s and Home Depot have similarly packaged 5-minute epoxy from other brands. Amazon has what looks like the equivalent from Loctite: 0.47 oz in a double syringe for $4.81.
  4. Walt’s (Bob Smith) 30-Minute Epoxy: At the hobby store. Amazon has it (with the Bob Smith label, of course) at $17.38 for 9 oz, but I paid under $10.

Clearly at the prices quoted above Elmer’s Carpenter’s Glue wins this event, though I think at Lowe’s it was the same size at the same price as Elmer’s Glue-All. Walt’s 30-Minute Epoxy is roughly a factor of 2 pricier (per ounce). By the nature of epoxy, you can count on wasting more of it than with white or yellow glue, so that pushes the price up more. But the 5-Minute Epoxy is in last place, and it’s not even close. Less than half an ounce for half of what I paid for nine ounces of Walt’s. Crazy!

Our second event is the usage event:

  1. Elmer’s Glue-All: You squeeze it on, you spread it around, you wipe off any excess, you’re good. Or you squeeze it onto some scrap paper, pick some up with a stick, apply it that way. No problem.
  2. Elmer’s Carpenter’s Glue: Likewise.
  3. Devcon 5-Minute Epoxy: You put on Nitrile gloves and make sure your ventilation is okay. You squeeze some out, watching to make sure you’re dispensing about the same amount from both tubes. You mix it together. You pick it up with a stick and apply it. You do small quantities and use it quickly before it sets. You toss out what you didn’t use right away.
  4. Walt’s (Bob Smith) 30-Minute Epoxy: Sort of likewise, but you have to squeeze out the resin and hardener separately and try to get equal amounts of both. A sensitive scale, 0.01 g, can help. I’ve seen this one recommended.

The winners are the two Elmer’s. The Devcon might seem easier than the Walt’s, but only if the syringes are really dispensing both sides equally; it’s difficult or impossible to fine tune the mix. You also need to use it faster.

Our third, and most important event, is the grab event. I carried these tests out last night. I took chunks of BT-60 tube and coupler tube, and glued the one into the other. Then I checked for how long they took to freeze up.

  1. Elmer’s Glue-All (thin film): I gave each of the Elmer’s glues two chances. On the first I used a thin film of glue. Hard to quantify; I’d say I squeezed out about a cm diameter glob on a piece of waxed paper, and used as much of that as I could pick up. It was just enough to put a thin layer around the inside of the BT-60. The Glue-All grabbed immediately. I had no way to make any adjustments after insertion.
  2. Elmer’s Carpenter’s Glue (thin film): Also grabbed immediately.
  3. Elmer’s Glue-All (thick application): For the second chance, I used 2 or 3 times as much glue. This made for a rather thick and messy joint, definitely more glue than needed to make a solid connection. The Glue-All remained loose enough to adjust for a minute or two.
  4. Elmer’s Carpenter’s Glue (thick application): Grabbed immediately.
  5. Devcon 5-Minute Epoxy (thin film): Remained loose for a minute or two. I’d say it was comparable to, maybe slightly looser than, the thick application of Elmer’s Glue-All. Given this, I didn’t bother with a thick application.
  6. Walt’s (Bob Smith) 30-Minute Epoxy: Remained loose for at least ten minutes. In fact, I came back after more than 30 minutes — perhaps close to an hour — and when I picked it up and twisted it, the coupler still moved — once. Then it froze up. The epoxy was still tacky. Next morning, however, it had cured fully. My guess is that I misjudged the hardener/resin ratio. This is where that 0.01 g scale would come in handy. Or perhaps atmospheric conditions were unfavorable?

It’s a little harder to pick winners here. The 30-minute epoxy certainly stayed far looser than anything else for far longer; possibly too long, and possibly in part due to mixing error, though I’m sure it would still have been the loosest if I’d mixed it exactly. You might not want a joint staying this loose for this long. In that case there’s a near tie between the 5 minute epoxy and the thick application of Elmer’s Glue-All. The latter makes for a heavier, uglier joint, but doesn’t require gloves and ventilation. I also don’t know how repeatable my experience was; if you’re careless and leave a small thin section on the glue application, will that grab up immediately? I’m inclined to give the nod to the 5-minute epoxy, if only for that reason plus the lighter, nicer joint. The yellow glue, of course, is a disaster.

Our final, very subjective event: Result quality. Not much to say here. It’s a tie. I don’t see much if any signs of Coke-bottling or other tube deformations on any of the joints. Certainly none as significant as the distortions arising from my trying to pry frozen joints loose. I have no quantitative way to measure the strength of the joints, but I’m confident they all are good enough for low power rocketry. Allegedly the 30 minute epoxy should be stronger than the 5 minute, but I doubt if it matters. As mentioned above, the thick Elmer’s applications result in heavier, messier (on the inside) joints.

Conclusions? Well, not so clear cut. I’m sure I don’t want to use Elmer’s Carpenter’s Glue again for couplers or motor mount installations. I’m inclined not to want to use Elmer’s Glue-All for these purposes either. If I did I’d certainly try to take care to use lots of it, spread uniformly. Depending on circumstances, I’d prefer one or another type of epoxy. If I wanted to leave the work undisturbed for an hour or more to cure, and if there were no issues of possibly slipping out of position, I’d go for the 30 minute, to maximize my ability to adjust the position and to maximize the joint strength. Otherwise the 5 minute would probably work fine. Though once I’ve used up the Devcon I’d buy the 5-minute Walt’s, or some other brand whose cost is more reasonable.

Again, I like the yellow glue fine for installing fins, building motor mounts, and so on. Fin fillets? Yet another story. I’d try Titebond No-Drip, No-Run Carpenter’s Glue, if I had any — so far I haven’t turned up a local source. I do have some Alleene’s Tacky Glue which has been recommended for fin fillets, and I’m going to give that a try. I think I’ll steer clear of epoxy for that purpose, though, unless those possibilities disappoint me.

Followup: I quote from http://www.rcgroups.com/forums/showthread.php?t=994056:

I wrote Bob Smith Industries about weighing instead of measuring their epoxies, rather quickly I got a very nice reply.  Yes, I use BSI epoxies, my local hobby shop carries it (I like to support brick and mortar stores), and they work rather well for this hobby 

The correct ratio for weighing is 1.1:1, as the resin is 10% heavier than the hardener. But in reality there is a built in “fudge factor” with their epoxy, so you can just go 1:1, as per Charlee Smith of BSI without any discernible differences.

So, not from the horse’s mouth but at one remove, this says 10% errors are OK.

I’ll still probably pick up one of those scales; at $15, why not? I’m sure I’ll find lots of uses.

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